In many industries such as aviation fabrication, the need arises to provide materials and structures which are light-weight and strong. While many light-weight and strong structures may be provided through the use of exotic metals and alloys, a great number of structures are required which can not support the expense generally encountered through the use of exotic, light-weight, high-strength metals. One of the most useful fabrication materials for providing structures which maintain high-strength while simultaneously being light in weight is found in so-called "honeycomb" metal panels. Such panels are usually formed of a pair of generally planar metal sheets often referred to as face sheets together with a matrix or core material. In most honeycomb panels, the matrix or core is formed of a generally corrugated structure fabricated of long thin metal strips which are multiply curved or faceted and periodically joined together to form a high-strength matrix or core. One of the most common core structures utilizes elongated thin metal strips which form a core structure generally resembling a section of a common beehive. The face sheets are joined to the edges of the core strips by attachments such as welding or adhesive attachment. Other forms of attachments such as braising, crimping or other means may be used with the overall objective being secure attachment between the matrix or core edges and the face sheets to provide a strong, light-weight and substantially rigid panel.
FIG. 1 sets forth a partially sectioned perspective view of a typical honeycomb constructed in accordance with conventional fabrication techniques and generally referenced by numeral 10. Panel 10 is shown to include a pair of generally planar metal sheets 11 and 12 formed of a material such as steel or other suitable metal. Panel 10 further includes a core 13 typically formed of a plurality of thin metal strips such as steel or the like multiply faceted and joined together to form a honeycomb-like structure. While not seen in FIG. 1, it will be understood in accordance with conventional fabrication techniques metal sheets 11 and 12 are commonly joined to their respective edges of core 13 by conventional fabrication such as welding, braising, adhesives or other attachment. For purposes of illustration, panel 10 is shown in a generally rectangular shape having opposed edges 14 and 15. It will be understood however that honeycomb panels of different shapes are often used in particular fabrications.
While honeycomb panels of the type shown in FIG. 1 provide an extremely cost effective and light-weight, high-strength fabrication material which is in many respects desirable for manufacturing processes, a substantial limitation has thus far arisen in the difficulty found in shaping such honeycomb panels to suit manufacturing needs. In many instances fabrications are provided in which the honeycomb panels are cut and welded together in sections to provide shapes which are generally angular or faceted in character. Unfortunately a substantial number of manufacturing needs such as those in aviation fabrication require that materials be formable into curved elements rather than faceted. The honeycomb core and face sheet combination of honeycomb panels does not allow the panels to be formed using conventional rolling or stamping processes due to the tendency of such processes to crush or deform the core material or overly stress the attachment of the core edges to the face sheets.
One of the most successful processes thus far developed for forming honeycomb panels into curved shapes for use in industries such as aviation fabrication is generally known as the "stretch wrap" or "stretch forming" process.
While stretch wrap or stretch forming processes have been developed in some variety, the basic stretch forming process utilizes an apparatus often referred to as "stretch wrap machine" in which the to-be-formed panel is positioned within a work station having a stationary support upon which a form block is secured. The form block defines a curved surface corresponding to the desired curvature which is to be imparted to the honeycomb panel. The remainder of the work station often referred to as a main frame supports a plurality of power driven articulated devices often referred to as articulated arms. Each supports a plurality of gripping jaws or other apparatus utilized in grasping the opposed edges of the honeycomb panel.
In some stretch wrap machines the jaw structures of the articulated arms may grip the panel edges directly. However, more often pluralities of elongated metal pull tabs are welded to each side of the honeycomb panel along opposed edges thereof. For the most part such pull tabs are usually arranged in pairs on each of the face sheets along the panel edges. The jaw structures of the articulated arms then grip the pluralities of pull tabs rather than the panel edges themselves. Once the panel is secured in the jaw structures within the stretch wrap machine, forces are applied to the articulated arms to draw the panel edges outwardly and thereby place the honeycomb panel in a predetermined tension. This tension is generally transverse to the major axes of the curved face of the form block. Once sufficient tension has been imposed upon the honeycomb panel, the articulated arms are then driven toward and beyond the form block to literally wrap it partially about the curved face of the form block. The wrapping process is carried forward while maintaining the panel in tension. It has been found that the maintenance of substantial tension upon the honeycomb panel during the wrapping process allows the panel to be formed into a curved shape without damaging the panel. The limitation on this process being care to ensure that the compressive strength of the core is not met or exceeded.
Once the wrapping process is complete, the tension applied to the panel is released and the panel curvature remains. Usually as the tension is relaxed a small amount of recovery or "spring back" occurs in the panel. However, for the most part, the panel retains its curvature.
For purposes of illustration, a perspective view of a simplified stretch wrap apparatus and honeycomb panel in FIG. 2. Thus, FIG. 2 shows a conventional stretch wrap machine generally referenced by numeral 20 having a stationary spine 16 supporting a form block 17. Form block 17 defines a curved surface 18 which in the illustration of FIG. 2 is a cylindrical segment. Stretch wrap machine 20 further includes pluralities of gripping jaws 23 and 24 supported by a pair of articulated arms 25 and 26 respectively. Arms 25 and 26 as well as pluralities of jaws 23 and 24 are fabricated in accordance with conventional fabrication techniques which are known in the art. While not seen in FIG. 2, it will be understood that conventional apparatus are utilized in stretch wrap machine 20 for supporting spine 16 and for articulating arms 25 and 26. Further, gripping jaw pluralities 23 and 24 are conventional and include conventional means for gripping (not shown). In accordance with conventional stretch wrap machine forming processes a honeycomb panel 10 having edges 14 and 15 is positioned within stretch wrap machine 20 and is supported by a pair of pull tabs 22 along edge 14 and a pair of pull tabs 21 along edge 15. As described above, pull tab pairs 21 and 22 are typically arranged on each side of one edge of panel 10 and are typically joined to the panel edge portions by welding attachment or the like. As is also mentioned above, jaw pluralities 23 and 24 grip the outwardly extending ends of pull tab pairs 21 and 22.
In operation, and as is described above, stretch wrap machine 20 utilizing power drive apparatus of conventional fabrication (not shown) draws arms 25 and 26 outwardly from edges 14 and 15 in the directions indicated by arrows 27 and 28. As a result, panel 10 is placed in tension in a force direction which is generally transverse to the major axes or axes of elongation of curved face 18 of form block 17. Once the predetermined tension has been applied to panel 10, the operating means driving arms 25 and 26 (not shown) force panel 10 rearwardly against curved surface 18 of form block 17 wrapping panel 10 against curved surface 18 in the directions indicated by arrows 29 and 30. The combination of tension applied to panel 10 and rearward force of arms 25 and 26 wraps panel 10 about curved surface 18 and beyond imposing a curvature upon panel 10 which corresponds generally to the curvature of curved surface 18.
Once the desired curvature has been imposed upon panel 10, the forces applied to panel 10 are released and the now curved panel corresponding to the dash-line representation in FIG. 2 is removed from the stretch wrap machine. Thereafter, excess material is cut from the curved panel and a completed curved element is provided.
In a typical stretch wrap fabrication process, a closed form such as a cylindrical form or the like is fabricated using a plurality of curved segments. The plurality of segments are usually joined to form a closed object using a welding process or the like.
FIG. 3 sets forth an illustrative closed form fabricated in accordance with conventional fabrication techniques illustrated in FIGS. 1 and 2 and described above. In the illustration of FIG. 3, a cylindrical form 30 is fabricated of a plurality of curved segments 19, 29 and 31 respectively joined along welded seams 32, 33 and 34 to form a cylinder. It will be recognized that segment 19 is formed as illustrated in FIG. 2 from panel 10 once the excess material is removed from the edges thereof. It will be further recognized that segments 29 and 31 are substantially identical to segment 19 and thus will be understood to have been formed in the manner illustrated in FIG. 2. The result in the fabrication shown in FIG. 3 provides a light-weight, high-strength cylindrically formed structure.
Despite the advantages of the conventional stretch wrap forming process described above in fabricating curved structures from honeycomb panels, a significant limitation arises in that the conventional stretch wrap forming process above is unable to fabricate complexly compound curved shapes. Thus, while shapes such as the cylindrical form shown in FIG. 3 are relatively easy to fabricate, shapes having reverse direction curves such as segments defining S-Shaped curvatures have not heretofore been formable using conventional stretch wrap fabrication. Furthermore, complex curves having differing axes of curvature are not formable using presently available stretch wrap methods and apparatus. By way of illustration, a frequently occurring type of complexly compound curved shape is found in objects often described as "coke bottle" shapes. This shapes acquires its name from its resemblance to certain commercial beverage bottles of the type manufactured by the Coca cola bottling company. However, the persuasiveness of such shape characteristics has caused the so-called coke bottle shape to acquire its own meaning within aviation fabrication industries.
Because such shapes are often utilized in industries such as aviation fabrication, and because honeycomb panels represent an otherwise attractive and light-weight advantageous fabricating material, there arises a continuing need in the art for method and process which facilitates the forming of honeycomb metal panels into compound complexly curved shapes.